Display Device

ABSTRACT

A display device with high-definition, in which display unevenness due to a voltage drop in a wiring or display unevenness due to a variation in characteristics of TFTs are suppressed. The display device of the invention comprises a first wiring for transmitting a video signal and a second wiring for supplying a current to a light emitting element. The first wiring and the second wiring extend parallel to each other, and are formed so as to overlap with each other at least partly with an insulating layer interposed therebetween.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.13/211,815, filed Aug. 17, 2011, now allowed, which is a continuation ofU.S. application Ser. No. 11/774,401, filed Jul. 6, 2007, now U.S. Pat.No. 8,018,403, which is a continuation of U.S. application Ser. No.10/827,444, filed Apr. 20, 2004, now U.S. Pat. No. 7,250,720, whichclaims the benefit of a foreign priority application filed in Japan asSerial No. 2003-122988 on Apr. 25, 2003, all of which are incorporatedby reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an active matrix display device, andmore particularly to a wiring structure of an active matrix displaydevice comprising a light emitting element.

2. Description of the Related Art

In recent years, development of large-scale electroluminescence(hereinafter abbreviated to EL) display device has been advanced withthe intention of coming to the television market.

When wiring length increases according to the increase in size of adisplay device, such problem as a voltage drop arises. There is aproblem in that a voltage applied to each EL element varies with placeby generation of the voltage drop so that display unevenness is caused.

In the case where film thickness of the wiring is increased in order tosolve the above-mentioned problem, a lot of loads are applied to thesteps of forming film, etching and the like. In addition, in the casewhere line width of the wiring is increased, the area ratio that thewiring occupies on the substrate is increased. Thus it makes difficultto fabricate a display device with high-definition.

With the increase in size of a display device, particularly in an activematrix display device, a variation in characteristics of thin filmtransistors (hereinafter referred to as TFTs) for transmitting anelectric signal to an EL element on a substrate becomes large, leadingto display unevenness.

Aiming to reduce the display unevenness due to the variation incharacteristics of TFTs, a circuit for driving the EL element has beenconfigured taking it into consideration (e.g., Patent Document 1).However, by providing a circuit for compensating for the variation incharacteristics of TFTs, the ratio that the circuit occupies on thesubstrate is increased and the aperture ratio of a pixel portion isreduced.

[Patent Document 1]

-   Japanese Laid-Open Patent Application No. 2003-5710

As described above, it is difficult to achieve the high-definition of adisplay device and the suppression of display unevenness due to avoltage drop in wirings or due to a variation in characteristics of TFTsat the same time.

SUMMARY OF THE INVENTION

In view of the foregoing problem, an object of the present invention isto provide a display device with high-definition, in which displayunevenness due to a voltage drop in wirings or the one due to avariation in characteristics of TFTs is suppressed.

A display device of the invention comprises a first wiring fortransmitting a video signal and a second wiring for supplying a currentto a light emitting element. The first and the second wirings extendparallel to each other, and are formed so as to be overlapped at leastpartly with an insulating layer interposed therebetween. Note that, thelight emitting element has a structure in which a light emitting layeris sandwiched between a pair of electrodes.

The first and the second wirings may be overlapped so that the firstwiring is the upper or the first wiring is the lower.

The first and the second wirings are not necessarily overlappedentirely, but they may be overlapped partly.

Electrodes of the light emitting element may be formed on the same layeras the upper wiring, namely either the first wiring or the secondwiring. With this structure, a pixel electrode (an electrode of a pairof electrodes of the light emitting element, which is connected to acircuit for transmitting a signal to the light emitting element) can beformed without additionally forming an insulating layer. Consequently,the steps of forming film, opening a contact hole and the like aresimplified.

With the above-mentioned structure, in the case of suppressing a voltagedrop by increasing the width of the second wiring, the width can beincreased using efficiently either the upper part or the lower part ofthe surface occupied by the first wiring. Thus, decrease in the apertureratio due to increase in the width of the second wiring is suppressed asless as possible. In addition, short circuit generated between the firstand the second wirings can be reduced because the first and the secondwirings are formed on different layers.

A display device of the invention comprises a first wiring fortransmitting a video signal, a second wiring for supplying a current toa light emitting element, and a third wiring extending parallel to thefirst and the second wirings. The first and the second wirings areformed on the same layer, the third wiring is formed either over orunder the first and the second wirings so as to overlap at least partlywith either the first wiring or the second wiring with an insulatinglayer interposed therebetween, and the second wiring and the thirdwiring are connected to each other.

The first and the second wirings may be overlapped over the thirdwiring, or under the third wiring.

Electrodes of the light emitting element may be formed on the same layeras the upper wiring, namely either the first wiring or the third wiring.With this structure, a pixel electrode can be formed withoutadditionally forming an insulating layer. Consequently, the steps offorming film, opening a contact hole and the like are simplified.

As mentioned above, by providing the third wiring which overlaps atleast partly with the first wiring or the second wiring, a voltage dropof the second wiring is suppressed using efficiently either the upperpart or the lower part of the surface occupied by the first wiring orthe second wiring.

A display device of the invention comprises a first wiring fortransmitting a video signal, a second wiring for supplying a current toa light emitting element, and a third wiring extending parallel to thefirst wiring and the second wiring. The first wiring and the secondwiring are formed so as to overlap at least partly with an insulatinglayer interposed therebetween, the third wiring is formed so as tooverlap at least partly with either the first wiring or the secondwiring with an insulating layer interposed therebetween, and the secondwiring and the third wiring are connected to each other.

The first and the second wirings may be overlapped so that the firstwiring is the upper or the first wiring is the lower.

The first and the second wirings are not necessarily overlappedentirely, and they may be overlapped partly.

The third wiring may be overlapped over the first wiring, or under thefirst wiring. The third wiring may also be overlapped over the secondwiring, or under the second wiring.

Electrodes of the light emitting element may be formed on the same layeras the most upper wiring among the first wiring, the second wiring andthe third wiring. With this structure, a pixel electrode can be formedwithout additionally forming an insulating layer. Consequently, thesteps of forming film, opening a contact hole and the like aresimplified.

With the above-mentioned structure, a voltage drop generated in thesecond wiring can be more reduced.

As described above, according to the invention, a display device withhigh image quality and high-definition can be fabricated whilesuppressing display unevenness due to a voltage drop in wirings forsupplying a current to a light emitting element.

As another configuration, a display device of the invention comprises alight emitting element, a first transistor for determining a currentvalue for flowing to the light emitting element, a second transistor fordetermining whether the light emitting element emits light or notaccording to a video signal, a third transistor for controlling an inputof the video signal, a fourth transistor for making the light emittingelement in a non-light emitting state regardless of the video signal, afirst wiring connected to the third transistor and transmitting thevideo signal, a second wiring connected to the second transistor andsupplying a current to the light emitting element through the first andthe second transistors, and a third wiring connected to the gateelectrode of the first transistor. The first wiring, the second wiringand the third wiring extend parallel to each other, and the first wiringand the third wiring are formed on the same layer and overlapped withthe second wiring at least partly with an insulating layer interposedtherebetween.

With the above-mentioned configuration, display unevenness due to avariation in characteristics of TFTs and the one due to a voltage dropin the wiring for supplying a current to a light emitting element can besuppressed.

According to the invention, a display device with high image quality andhigh-definition can be obtained in which display unevenness due to avoltage drop in a wiring is suppressed. In addition, a display devicewith high image quality and high-definition can be obtained, in whichdisplay unevenness due to a voltage drop in a wiring and displayunevenness due to a variation in characteristics of TFTs are bothsuppressed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram describing one mode of the invention.

FIG. 2 is a diagram showing a circuit in a pixel portion.

FIG. 3 is a diagram describing one mode of the invention.

FIG. 4 is a diagram showing a circuit in a pixel portion.

FIG. 5 is a diagram describing one mode of the invention.

FIG. 6 is a schematic diagram showing an external circuit and a panel.

FIG. 7 is a configuration diagram of a signal line driver circuit.

FIGS. 8A to 8F are views of electronic apparatuses to which theinvention is applied.

FIG. 9 is a view describing one mode of the invention.

FIG. 10 is a view describing one mode of the invention.

FIG. 11 is a view describing one mode of the invention.

DETAILED DESCRIPTION OF THE INVENTION

Embodiment modes of the present invention will be explained withreference to the accompanying drawings hereinafter. However, it is to beunderstood that various changes and modifications will be apparent tothose skilled in the art. Therefore, unless otherwise such changes andmodifications depart from the scope of the present invention, theyshould be construed as being included therein.

Embodiment Mode 1

An embodiment mode of the invention will be explained with reference toFIG. 1.

FIG. 1 is a top plan view of a pixel portion of a display device towhich the invention is applied. FIG. 9 is a cross-sectional view cuttingalong a line A-A′ of FIG. 1.

In FIG. 1, provided are a source signal line 101 (101 a, 101 b) as awiring for transmitting a video signal and a current supply line 104(104 a, 104 b) as a wiring for supplying a current to a light emittingelement. The source signal line 101 and the current supply line 104 areformed on different layers with an insulating layer interposedtherebetween to overlap with each other. In addition, they extendparallel to each other. Note that, although the entire source signalline 101 and the current supply line 104 are overlapped with each otherin this embodiment mode, a part of the source signal line 101 and a partof the current supply line 104 may be overlapped. In any case, the widthof the current supply line 104 can be increased by using the upper partof the source signal line 101. In addition, in this embodiment mode, thecurrent supply line 104 is provided over the source signal line 101,however, the invention is not limited to this structure, and the currentsupply line 104 may be provided under the source signal line 101.

In addition to the source signal line 101 and the current supply line104, a driving TFT 110 for determining whether the light emittingelement emits light or not according to a video signal, a switching TFT111 for controlling the input of the video signal, and an erasing TFT112 for making the light emitting element in a non-light emitting stateregardless of the video signal are provided in the pixel portion.

In this embodiment mode, the current supply line 104 is connected to thedriving TFT 110 through a conductive layer 120 (120 a, 120 b) which isformed on the same layer as the source signal line 101. A part of afirst gate signal line 102 functions as the gate electrode of theswitching TFT 111. A part of a second gate signal line 103 (103 a, 103b) functions as the gate electrode of the erasing TFT 112. In addition,the driving TFT 110 is connected to a first electrode 130 (130 a, 130 b,130 c) of the light emitting element through a conductive layer 121 (121a, 121 b) which is formed on the same layer as the source signal line101. Being not shown in FIG. 1, a bank having an opening portion isformed so as to expose the first electrode 130 of the light emittingelement, an electroluminescent layer, and a second electrode of thelight emitting element. An overlapping area of the first electrode 130of the light emitting element, the electroluminescent layer and thesecond electrode of the light emitting element functions as the lightemitting element.

In FIG. 9, reference numeral 51 denotes a current supply line, 52denotes a source signal line, 53 denotes a first electrode of a lightemitting element, 54 denotes the light emitting element, 55 denotes asemiconductor layer, 56 denotes a gate electrode, 57 denotes a bank, 58and 59 denote insulating layers, 60 denotes a protective film, and 61denotes a second electrode of the light emitting element.

FIG. 2 shows a circuit configuration of the pixel portion shown in thisembodiment mode. In this embodiment mode, a driving TFT 210, a switchingTFT 211, and an erasing TFT 212 are provided, however, a circuitconfiguration comprising only the driving TFT 210 and the switching TFT211, or a circuit configuration comprising other TFT or wiring may beapplied. That is, the circuit configuration of the invention is notlimited to the one shown in this embodiment mode.

Furthermore, a thin film transistor (TFT) is used in this embodimentmode, however, a transistor fabricated by using a silicon wafer of bulkor an SOI (Silicon On Insulator) may also be used. As the structure ofthe transistor, both a single gate structure and a multi-gate structurein which a plurality of gates are provided may be employed. A top gatestructure and a bottom gate structure may be employed as well.

By applying the invention, the width of a current supply line can beincreased by using efficiently the upper part or the lower part of asurface occupied by a source signal line, and a voltage drop in thecurrent supply line can be suppressed. Consequently, particularly in adisplay device of a lower surface emitting type or a dual emitting type,the decrease in the aperture ratio due to the increase in the width ofthe current supply line can be suppressed as less as possible. As aresult, a display device can be fabricated having little displayunevenness due to a voltage drop and capable of displaying withhigh-definition. In addition, as the source signal line and the currentsupply line are formed on different layers, short circuit generatedbetween the source signal line and the current supply line can bereduced and a display device with high image quality can be fabricated.The productive yield of a display device is enhanced.

Embodiment Mode 2

An embodiment mode of the invention will be explained with reference toFIG. 3.

FIG. 3 is a top plan view of a pixel portion of a display device towhich the invention is applied. FIG. 10 is a cross-sectional viewcutting along a line A-A′ of FIG. 3.

In FIG. 3, a source signal line 301 (301 a, 301 b) as a wiring fortransmitting a video signal and a current supply line 305 (305 a, 305 b)as a wiring for supplying a current to a light emitting element areprovided. The source signal line 301 and the current supply line 305 areformed on the same layer and extend parallel to each other. In addition,above the source signal line 301 and the current supply line 305, awiring 304 is formed with an insulating layer interposed therebetween.The wiring 304 extends parallel to the source signal line 301 or thecurrent supply line 305. The wiring 304 and the current supply line 305are connected to each other through a contact hole. Note that, in thisembodiment mode, a part of the source signal line 301 and the entirecurrent supply line 305 are overlapped with the wiring 304. However, apart of the source signal line 301 and a part of the current supply line305 may be overlapped with the wiring 304, or the entire source signalline 301 and the entire current supply line 305 may be overlapped withthe wiring 304. In any case, the voltage drop in the current supply line305 can be suppressed by the wiring 304 which is provided by using theupper part of the current supply line 305 and connected to the currentsupply line 305. Further, in this embodiment mode, the wiring 304 isprovided over the source signal line 301 and the current supply line305, though the invention is not limited to this structure, and thewiring 304 may be provided under the source signal line 301 and thecurrent supply line 305.

In addition to the source signal line 301 and the current supply line305, a driving TFT 310 for determining whether the light emittingelement emits light or not according to a video signal, a switching TFT311 for controlling the input of the video signal, and an erasing TFT312 for making the light emitting element in a non-light emitting stateregardless of the video signal are provided in the pixel portion.

In FIG. 10, reference numeral 30 denotes a current supply line, 31denotes a wiring, 32 denotes a source signal line, 33 denotes a firstelectrode of a light emitting element, 34 denotes the light emittingelement, 35 denotes a semiconductor layer, 36 denotes a gate electrode,37 denotes a bank, 38 and 39 denote insulating layers, 40 denotes aprotective film, and 41 denotes a second electrode of the light emittingelement.

In this embodiment mode, a part of a first gate signal line 302functions as the gate electrode of the switching TFT 311. A part of asecond gate signal line 303 functions as the gate electrode of theerasing TFT 312. In addition, the driving TFT 310 is connected to afirst electrode 330 (330 a, 330 b, 330 c) of the light emitting elementthrough a conductive layer 321 (321 a, 321 b) which is formed on thesame layer as the source signal line 301. Being not shown in FIG. 3, abank having an opening portion formed so as to expose the firstelectrode 330 of the light emitting element, an electroluminescentlayer, and a second electrode of the light emitting element are formed.An overlapping area of the first electrode 330 of the light emittingelement, the electroluminescent layer and the second electrode of thelight emitting element functions as the light emitting element.

FIG. 2 shows a circuit configuration of the pixel portion shown in thisembodiment mode. Although the driving TFT 210, the switching TFT 211,and the erasing TFT 212 are provided in this embodiment, however, thecircuit configuration comprising only the driving TFT 210 and theswitching TFT 211, or the circuit configuration comprising other TFT orwiring may also be applied. That is, the circuit configuration of theinvention is not limited to the one shown in this embodiment mode.

Furthermore, a thin film transistor (TFT) is used in this embodimentmode, however, a transistor fabricated by using a silicon wafer of bulkor an SOI (Silicon On Insulator) may also be used. As the structure ofthe transistor, both a single gate structure and a multi-gate structurein which a plurality of gates are provided may be employed. A top gatestructure and a bottom gate structure may be employed as well.

By applying the invention, the width of a current supply line can beincreased by using efficiently the upper part or the lower part of asurface occupied by the source signal line and a voltage drop in thecurrent supply line can be suppressed. Consequently, particularly in adisplay device of a lower surface emitting type or a dual emitting type,the decrease in the aperture ratio due to the increase in the width ofthe current supply line can be suppressed as less as possible. As aresult, a display device can be fabricated having little displayunevenness due to a voltage drop and capable of displaying withhigh-definition.

Embodiment Mode 3

In each of the display devices shown in Embodiment Mode 1 and EmbodimentMode 2, the current supply line 104 or the wiring 305 are provided onthe same layer as the first electrode 130 or 330 of the light emittingelement respectively.

However, the structure of the display device of the invention is notlimited to this, the first electrode 130 or 330 of the light emittingelement may be provided over the current supply line 104 or the wiring305 respectively with an insulating layer interposed therebetween. Withsuch a structure, particularly in a display device of a upper surfaceemitting type, the opening part can be designed with more flexibilityand the aperture ratio is improved.

In addition, in the fabricating process of a display device, aflattening process after the formation of a transparent conductive layerfor forming the first electrodes 130 and 330 of the light emittingelement may be simplified.

Embodiment 1

In this embodiment, a configuration and a driving method of a pixelportion of a display device to which the invention applied will beexplained.

In FIG. 4, a source signal line 701 (701 a, 701 b) as a wiring fortransmitting a video signal and a current supply line 704 (704 a, 704 b)as a wiring for supplying a current to a light emitting element areprovided. The source signal line 701 and the current supply line 704 areformed on different layers with an insulating layer interposedtherebetween to overlap with each other and extend parallel to eachother. A power supply line 705 is provided on the same layer as thesource signal line 701 and extends parallel to the source signal line701. The entire of the source signal line 701 and the power supply line705 overlap with the current supply line 704. The current supply line704 with enough long width and less voltage drop is formed by using theupper part of the source signal line 701 and the power supply line 705.

In this embodiment, the current supply line 704 is provided under thesource signal line 701 and the power supply line 705, however, theinvention is not limited to this structure, and the current supply line704 may be provided over the source signal line 701. The current supplyline 704 may be overlapped with only a part of the source signal line701 or the power supply line 705.

In addition to the source signal line 701 and the current supply line704, a current controlling TFT 711 for determining a current valueflowing to the light emitting element, a driving TFT 710 for determiningwhether the light emitting element emits light or not according to avideo signal, a switching TFT 712 for controlling the input of the videosignal, and an erasing TFT 713 for making the light emitting element ina non-light emitting state regardless of the video signal are providedin the pixel portion. The current controlling TFT 711 is formed so thatthe L/W (channel length/channel width) is larger than the one of thedriving TFT 710 and an active layer has crooked shape.

FIG. 11 is a cross-sectional view cutting along a line A-A′ of FIG. 4.In FIG. 11, insulating layers 18 and 19 are formed with an organic film.A nitride film formed by sputtering is provided on the insulating layer18. Note that, the insulating layers 18 and 19 may be formed with aninorganic film such as a silicon oxide film as well as the organic film.

The source signal line 701 is connected to the switching TFT 712 througha conductive layer 720 (720 a, 720 b) which is formed on the same layeras the current supply line 704. A part of a first gate signal line 702functions as the gate electrode of the switching TFT 712. A part of asecond gate signal line 703 functions as the gate electrode of theerasing TFT 713. Further, the power supply line 705 is connected to thegate electrode of the current controlling TFT 711. The currentcontrolling TFT 711 is connected to a first electrode 730 (730 a, 730 b,730 c) of the light emitting element through a conductive layer 720which is formed on the same layer as the source signal line 701. Thefirst electrode 730 of the light emitting element and the current supplyline 701 are formed on the same layer. Being not shown in FIG. 4, a bankhaving an opening portion formed so as to expose the first electrode 730of the light emitting element, an electroluminescent layer, and acathode are formed. An overlapping area of the first electrode 730 ofthe light emitting element, the electroluminescent layer and the secondelectrode of the light emitting element functions as the light emittingelement.

In FIG. 11, reference numeral 10 denotes a current supply line, 11denotes a power supply line, 12 denotes a source signal line, 13 denotesa first electrode of a light emitting element, 14 denotes the lightemitting element, 15 denotes a semiconductor layer, 16 denotes a gateelectrode, 17 denotes a bank, 18 and 19 denote insulating layers, 20denotes a protective film, and 21 denotes a second electrode of thelight emitting element.

FIG. 5 shows a circuit configuration of the pixel portion shown in thisembodiment.

In FIG. 5, a p-channel transistor is used for a driving TFT 811 and acurrent controlling TFT 810, and the drain of the current controllingTFT 810 and an anode of a light emitting element 840 are connected toeach other. In this embodiment, the first electrode 730 of the lightemitting element functions as an anode and the second electrode of thelight emitting element functions as a cathode. On the other hand, in thecase where an n-channel TFT is used for the driving TFT 811 and thecurrent controlling TFT 810, the source of the current controlling TFT810 and a cathode of the light emitting element 840 are connected toeach other. In this case, the first electrode 730 of the light emittingelement functions as a cathode and the second electrode of the lightemitting element functions as an anode.

A driving method of the pixel shown in FIG. 5 will be explained next.The operation of the pixel shown in FIG. 5 can be explained by dividinginto a writing period and a holding period. Firstly, a first gate signalline 802 is selected in the writing period, thereby turning on aswitching TFT 812 whose gate is connected to the first gate signal line802. Then a video signal inputted to a source signal line 801 isinputted to the gate of the driving TFT 811 through the switching TFT812. Note that, the current controlling TFT 810 is always turned on asthe gate is connected to a power supply line 805.

In the case where the driving TFT 811 is turned on by a video signal, acurrent is supplied to the light emitting element 840 through thecurrent supply line 804. In this embodiment, the driving TFT 811operates in a linear region, and a current flowing to the light emittingelement 840 is determined according to voltage-current characteristicsof the current controlling TFT 810 which operates in a saturation regionand the light emitting element 840. The light emitting element 840 emitslight with the brightness corresponding to the supplied current.

In the case where the current controlling TFT 810 is turned off by avideo signal, no current is supplied to the light emitting element 840and the light emitting element 840 does not emit light.

Secondly, in a holding period, the switching TFT 812 is turned off bycontrolling a potential of the first gate signal line 802 and apotential of the video signal which has been written in the writingperiod is held. In the case where the driving TFT 811 is turned on inthe writing period, a current supplied to the light emitting element 840is kept as the potential of the video signal is held in a capacitor 814.On the other hand, in the case where the driving TFT 811 is turned offin the writing period, a current is not supplied to the light emittingelement 840. Note that, although the capacitor 814 is provided in acircuit in this embodiment, a circuit without capacitor is alsopossible.

In an erasing period, a second gate signal line 803 is selected and anerasing TFT 813 is turned on to apply a potential of the current supplyline 804 to the gate of the driving TFT 811 through the erasing TFT 813.Consequently, the driving TFT 811 is turned off, so that a compulsivestate in which a current is not supplied to the light emitting element840 can be produced.

In the above-mentioned configuration, the current controlling TFT 810operates in a saturation region. Therefore, the variation of a draincurrent of the current controlling TFT 810 is small against thevariation of a voltage between the source and the drain of the currentcontrolling TFT 810, and a current flowing to the light emitting element840 is less sensitive to a slight change in a voltage between the gateand the source of the driving TFT 811 (Vgs). The current flowing to thelight emitting element 840 is determined by the current controlling TFT810 which operates in a saturation region. Therefore, it realizes noinfluence on the current flowing to the light emitting element 840without increasing the capacitance of a capacitor 814 provided betweenthe gate and the source of the current controlling TFT 810 andsuppressing an OFF current of the switching TFT 812 to low. The currentflowing to the light emitting element 840 is also not sensitive to theparasitic capacitance in the gate of the driving TFT 811. As a result,the brightness variation due to the variation in characteristics of TFTsand the like is reduced and display unevenness can be reduced.

As for the light emitting element 840, the first electrode 730 and thesecond electrode are formed with a transparent conductive layer in thisembodiment. Therefore, a light can be received from both sides, theupper surface and the lower surface (the side on which the TFT is formedis referred to as the lower surface, and the opposite side thereof isreferred to as the upper surface with the electroluminescent layerinterposed therebetween). Note that, the display device of the inventionis not limited to this structure, the structure of receiving a lightfrom either the upper surface or the lower surface may be applied.

By applying the invention, the width of the current supply line can beincreased by using the lower part of a surface occupied by the sourcesignal line and the power supply line and a voltage drop in the currentsupply line can be suppressed. Consequently, in a display device ofreceiving a light from a lower surface such as the one shown in thisembodiment, the decrease in the aperture ratio due to the increase inthe width of the current supply line can be suppressed as less aspossible. As a result, a display device can be fabricated having littledisplay unevenness due to a voltage drop and capable of displaying withhigh-definition. In addition, by applying a circuit configuration shownin this embodiment, display unevenness due to the variation incharacteristics of TFTs can be suppressed and a display image with highquality is achieved.

Embodiment 2

In this embodiment, a structure and a driving method of an active matrixdisplay device comprising the pixel portion shown in Embodiment 1 willbe explained.

A block diagram of an external circuit and a schematic diagram of apanel are shown in FIG. 6.

As shown in FIG. 6, the active matrix display device to which thepresent invention is applied comprises an external circuit 3004 and apanel 3010. The external circuit 3004 comprises an A/D converter 3001, apower supply portion 3002, and a signal generating portion 3003. In theA/D converter 3001, an analog video data signal is converted into adigital video data signal to supply it to a signal driver circuit 3006.In the power supply portion 3002, power sources each of which has adesired voltage value are generated by power sources supplied from abattery or a socket and supplied to the signal driver circuit 3006, ascan driver circuit 3007, an OLED element 3011, the signal generatingportion 3003 and the like. The power source, the video signal, asynchronous signal and the like are inputted to the signal generatingportion 3003 and each signal is converted, and a clock signal and thelike for driving the signal driver circuit 3006 and the scan drivercircuit 3007 are generated therein.

The signals and the power sources from the external circuit 3004 areinputted to an internal circuit and the like from an FPC connectionportion 3005 within a panel through an FPC.

The panel 3010 comprises the FPC connection portion 3005 and theinternal circuit, which are disposed on a grass substrate 3008, and theOLED element 3011. The internal circuit comprises the signal drivercircuit 3006, the scan driver circuit 3007, and a pixel portion 3009.Although the pixel described in Embodiment Mode 1 is applied to the onein FIG. 6 as an example, any one of pixel configurations described inembodiment modes of the invention may be applied to the pixel portion3009.

The pixel portion 3009 is disposed in the center of the substrate, andthe signal driver circuit 3006 and the scan driver circuit 3007 aredisposed around the pixel portion 3009. A plurality of the OLED elements3011 and a counter electrode of the light emitting elements are formedover the whole surface of the pixel portion 3009.

A block diagram of the signal driver circuit 3006 is shown in FIG. 7 inmore details.

The signal driver circuit 3006 comprises a shift register 4002 which iscomposed of plural stages of D flip-flops 4001, data latch circuits4003, latch circuits 4004, level shifters 4005, and buffers 4006 and thelike.

Signals to be inputted to the signal driver circuit 3006 are a clocksignal (S-CK), an inverted clock signal (S-CKB), a start pulse (S-SP), adigital video signal (DATA), and a latch pulse (LatchPulse).

First, the shift register 4002 outputs a sampling pulse sequentially intiming with the clock signal, the inverted clock signal, and the startpulse. The sampling pulse is inputted to the data latch circuit 4003,where the digital video signal is taken in and held in response to theinput of the sampling pulse. This operation is conducted in orderstarting from the first column.

After the digital video signal is held in the data latch circuit 4003 atthe last column, the latch pulse is inputted during the horizontalretrace period to transmit the digital video signals held in the datalatch circuits 4003 to the latch circuits 4004 simultaneously. Then, thedigital video signal is level-shifted by the level shifter 4005 andrectified by the buffer 4006 before being outputted simultaneously tosignal lines S1 to Sn. Accompanied by this output, H level/L level isinputted to a pixel in a row selected by the scan driver circuit 3007 tocontrol whether the OLED element 3011 emits light or not.

The active matrix display device shown in this embodiment comprises thepanel 3010 and the external circuit 3004 which are provided separately.The panel and the external circuit may be integrally formed on the samesubstrate. In addition, the OLED element is used in the display device,however, another light emitting element may be used as well as the OLEDelement in a light emitting device. The level shifter 4005 and thebuffer 4006 are not necessarily provided in the signal driver circuit3006.

Embodiment 3

Electronic apparatuses to which the present invention is applied will beexplained in this embodiment. A display device using the inventionrealizes display with high quality images and high-definition by beingmounted on various electronic apparatuses. In addition, it can bemounted on small electronic apparatuses such as a mobile phone as wellas on a large display device such as a television.

FIG. 8A is a display device which includes a housing 5501, a supportbase 5502, and a display portion 5503. The invention is applicable to adisplay device having the display portion 5503.

FIG. 8B is a video camera which includes a body 5511, a display portion5512, a sound input portion 5513, operation switches 5514, a battery5515, and an image receiving portion 5516.

FIG. 8C is a notebook personal computer to which the invention isapplied and which includes a body 5501, a housing 5502, a displayportion 5503, and a keyboard 5504.

FIG. 8D is a personal data assistant (PDA) to which the invention isapplied. A body 5531 includes a display portion 5532, an externalinterface 5535, operation buttons 5534 and the like. In addition, astylus 5532 is provided as an attachment for the operation.

FIG. 8E is a digital camera which includes a body 5551, a displayportion (A) 5552, an eye contacting portion 5553, an operation switch5554, a display portion (B) 5555, and a battery 5556.

FIG. 8F is a mobile phone to which the invention is applied. A body 5561includes a display portion 5564, a sound output portion 5562, operationswitches 5565, and an antenna 5566.

1. A display device comprising: a first transistor, a second transistor,and a third transistor; a first insulating layer over the firsttransistor, the second transistor and the third transistor; a first lineelectrically connected to the first transistor; a second lineelectrically connected to the second transistor; a third lineelectrically connected to the third transistor; a pixel electrode overthe first insulating layer; a second insulating layer covering an endportion of the third line and an end portion of the pixel electrode; anEL layer over the second insulating layer; a cathode over the secondinsulating layer, wherein a part of the third line overlaps a part ofthe first line, wherein a part of the third line is in parallel with apart of the first line, wherein a part of the third line is in parallelwith a part of the second line, wherein the pixel electrode is incontact with the first insulating layer, and wherein the third line isin contact with the first insulating layer.
 2. The display deviceaccording to claim 1, wherein the pixel electrode is provided by using amaterial which differs from a material used for providing the thirdline.
 3. The display device according to claim 1, wherein the first lineis formed on the same layer as the second line.
 4. The display deviceaccording to claim 1, wherein the first line is a current supply linefor an EL element, the EL element including the EL layer.
 5. The displaydevice according to claim 1, wherein the second line is a video signalline.
 6. The display device according to claim 1, wherein the firsttransistor is driving transistor of an EL element, the EL elementincluding the EL layer.
 7. The display device according to claim 1,wherein the third line is directly connected to a gate of the thirdtransistor.
 8. The display device according to claim 1, wherein a sourceor a drain of the first transistor is directly connected to the firstline.
 9. The display device according to claim 1, wherein the third lineis a power supply line.
 10. A display device comprising: a firsttransistor, a second transistor, and a third transistor; a firstinsulating layer over the first transistor, the second transistor andthe third transistor; a first line electrically connected to the firsttransistor; a second line electrically connected to the secondtransistor; a third line electrically connected to the third transistor;a pixel electrode over the first insulating layer; a second insulatinglayer covering an end portion of the third line and an end portion ofthe pixel electrode; an EL layer over the second insulating layer; acathode over the second insulating layer; an opening of the secondinsulating layer over the pixel electrode, wherein a part of the ELlayer is provided in the opening, wherein a part of the cathode isprovided in the opening, wherein a part of the third line overlaps apart of the first line, wherein a part of the third line is in parallelwith a part of the first line, wherein a part of the third line is inparallel with a part of the second line, wherein the pixel electrode isin contact with the first insulating layer, and wherein the third lineis in contact with the first insulating layer.
 11. The display deviceaccording to claim 10, wherein the pixel electrode is provided by usinga material which differs from a material used for providing the thirdline.
 12. The display device according to claim 10, wherein the firstline is formed on the same layer as the second line.
 13. The displaydevice according to claim 10, wherein the first line is a current supplyline for an EL element, the EL element including the EL layer.
 14. Thedisplay device according to claim 10, wherein the second line is a videosignal line.
 15. The display device according to claim 10, wherein thefirst transistor is driving transistor of an EL element, the EL elementincluding the EL layer.
 16. The display device according to claim 10,wherein the third line is directly connected to a gate of the thirdtransistor.
 17. The display device according to claim 10, wherein asource or a drain of the first transistor is directly connected to thefirst line.
 18. The display device according to claim 10, wherein thethird line is a power supply line.
 19. A display device comprising: afirst transistor, a second transistor, and a third transistor; a firstinsulating layer over the first transistor, the second transistor andthe third transistor; a first line electrically connected to the firsttransistor; a second line electrically connected to the secondtransistor; a third line electrically connected to the third transistor;a pixel electrode over the first insulating layer; a second insulatinglayer covering an end portion of the third line and an end portion ofthe pixel electrode; an EL layer over the second insulating layer; acathode over the second insulating layer, wherein a part of the thirdline overlaps a part of the first line, wherein a part of the third lineis in parallel with a part of the first line, wherein a part of thethird line is in parallel with a part of the second line, wherein thepixel electrode is in contact with the first insulating layer, whereinthe third line is in contact with the first insulating layer, andwherein the third line is a power supply line.
 20. The display deviceaccording to claim 19, wherein the pixel electrode is provided by usinga material which differs from a material used for providing the thirdline.
 21. The display device according to claim 19, wherein the firstline is formed on the same layer as the second line.
 22. The displaydevice according to claim 19, wherein the first line is a current supplyline for an EL element, the EL element including the EL layer.
 23. Thedisplay device according to claim 19, wherein the second line is a videosignal line.
 24. The display device according to claim 19, wherein thefirst transistor is driving transistor of an EL element, the EL elementincluding the EL layer.
 25. The display device according to claim 19,wherein the third line is directly connected to a gate of the thirdtransistor.
 26. The display device according to claim 19, wherein asource or a drain of the first transistor is directly connected to thefirst line.